Abu Qatadah Al-Ansari (RAA) narrated, 'The Messenger of Allah (ﷺ) was asked about fasting on the day of Arafah (the 9th of the month of Dhul Hijjah). He replied, "Fasting on the day of Arafah is an expiation for the preceding year and the following year." He was also asked about fasting on the day of Ashura (the 10th of the month of Muharram). He replied, "Fasting on the day of Ashura is an expiation for the preceding year." The Messenger of Allah (ﷺ) was also asked about fasting on Monday, and he replied, "This is the day on which I was born and the day on which I was sent (with the Message of Islam) and the day on which I received revelation." Related by Muslim.
English reference: Book 5, Hadith 700
Arabic reference: Book 5, Hadith 680